1. Field of the Invention
This invention relates generally to an optical system. More particularly, it relates to an optical reduction system used in semiconductor manufacturing.
2. Related Art
Semiconductors are typically manufactured using various photolithographic techniques, which are implemented using complex optical systems. For example, one complex optical system used in the manufacturing of semiconductors is a catadioptric optical reduction system. While these complex optical systems perform adequately for their intended purpose, controlling aberrations in these optical systems is becoming increasingly important as semiconductor manufacturers strive to make semiconductor components smaller. Aberrations limit the minimum size of component features that may be reproduced on a semiconductor chip.
Imaging in complex optical systems can be controlled using multiple lenses and one or more mirrors. To reproduce small component features on a semiconductor using such systems, a passive device can be used to apply a force to a mirror of the optical system and thereby compensate for some of the aberrations in the optical system. For example, a passive device attached to a mirror can be used to compensate for at least some of the astigmatism in an optical system with relative success. Known passive devices have limitations, however, and they do not provide an element of control needed to compensate for changes in astigmatism and other aberrations that result from environmental factors, such as changes in temperature.
What is needed are new means for controlling aberrations in complex optical systems that permit such systems to be used for reproducing very small component features on a semiconductor.
The present invention provides an apparatus, system, and method for active compensation of aberrations in an optical system. In a preferred embodiment, first and second force bars are attached to a mirror. The first force bar is bifurcated to form an opening near its longitudinal midpoint. This opening defines first and second opposed surfaces. The second force bar is substantially perpendicular to the first force bar and extends through the opening of the first force bar so that a medial portion of the second force bar is disposed in the opening of the first force bar. The second force bar is connected to the first surface by at least one actuator. Longitudinal movement of the actuator applies a force to the mirror causing a displacement or a change in the shape of the mirror. A support structure is used to support the weight of the force bars and actuators. The force bars are connected to the support structure by a plurality of flexures.
In a preferred embodiment of the invention, the force bars are leaf springs and the actuators are pneumatic bellows. A sensing module for measuring a parameter used to control aberrations in the optical system provides information to a control module. The control module receives information from the sensing module and controls an air supply module to operate the pneumatic bellows.